(a) Field of the Invention
The present invention concerns the production of oxygen by adsorption of nitrogen from air, of the type where, on a plurality n of columns of adsorbent material, there are successively provided in a cyclical manner on one of said columns with time lag T/n from one column to the next one, T being the period of the cycle;
a) a step of production of oxygen of duration x by gas withdrawal in a so called co-current direction, from a column of adsorbent material of the zeolite type, under high pressure comprising a maximum cycle pressure PM, with at least partial air admission during this step;
b) a step of pumping of duration y in counter-current under sub-atmospheric pressure, to provide a pressure reduction, which pumping is continued possibly, during flushing by counter-current passage of oxygen enriched gas, the minimum cycle pressure reached during said step being Pm;
c) a step of repressurization incorpororating at the latest before the step of pumping, at least one phase of counter-current repressurization with oxygen enriched gas.
(b) Description of Prior Art
This procedure, intended for the industrial production of oxygen by fractionating air over zeolites, for example of the 5A or 13X type, gives oxygen enriched air with oxygen content up to 95%, the residual 5% essentially consisting of argon.
In a large number of applications, a production quality with a 90/93% oxygen content is sufficient. In this range of contents, the quantities of oxygen required by the application may range from a few tons/day to hundred of tons/day.
The process mentioned above was developped in the range of 10 to 50 T/day of oxygen, and it has shown to be very competitive cost wise with respect to the oxygen obtained by cryogenic means and supplied in liquid form, or by piping.
The different types of cycles proposed for the production of oxygen generally comprise from two to four adsorbers only one adsorber being in production, while the other (or the others) are either being regenerated, or in intermediate phase (recycling, repressurization . . . ).
Since the cycles have a duration which generally varies between 90 seconds and a few minutes, the size of the adsorbers for a given cycle of a predetermined duration, and with the same type of adsorbent material, increases in proportion to the flow to be produced. The compliance with the rules governing the speed of passage of the gas during certain phases requires a minimum section for the flow of gas which, for large sizes, directly or indirectly becomes a limiting factor. In the case of vertical cylindrical adsorbers and with vertical circulation of gas, the diameter of the adsorbers becomes excessive beyond a certain size of the apparatus (limitation of the diameters of the bottoms and sleeves, problems of transportation, etc . . . ).
In the case of horizontal cylindrical adsorbers with vertical circulation of gas, which enable to carry higher flows than in the preceding case, with the same diameter, the fact of passing large flows causes problems of gas distribution in the internal collectors on both sides of the adsorbent material, as well as an important increase of the dead volume in these collectors. Thus, it is believed that such a unit is limited to about 60 T/day.
In the case where the application requires more important quantities of over-oxygenated air and oxygen, for example 300 T/day, the actual solution is, either to install a plurality of units mounted in parallel (for example 3 units of 50 T/day each for a demand of 150 T/day), or use the solution with cryogenic means.
The aim of the present invention is to exceed the actual tonnage limits of oxygen produced per production unit. More specifically, the object is to produce, with a single unit, a quantity of oxygen which could be much higher than 60 T/day, which in the production cost, would decrease the portion of fixed expenses (civil engineering, engineering in general, installation, starting). Another object of the invention is to increase the productivity so as to still reduce the production cost as compared to the utilization of a plurality of production units, the increase of productivity representing a decrease of the investment (material, adsorbent material . . . ). Still another object of the invention is to reduce the energy consumption which will still reduce the cost for the production of oxygen.